Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reissue Patent
2000-07-19
2002-06-11
Shippen, Michael L. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reissue Patent
active
RE037744
ABSTRACT:
The present invention relates to a process for the production of vinyl acetate by contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst.
The preparation of supported palladium catalysts for the production of vinyl acetate generally involves impregnating a suitable support with a palladium compound followed by conversion of the palladium compound to substantially metallic palladium.
Methods for the preparation of shell-impregnated catalysts are described, for example, in U.S. Pat. No. 3,822,308, U.S. Pat. No. 4,048,096, U.S. Pat. No. 5,185,308, U.S. Pat. No. 5,332,710, CA 2128162, U.S. Pat. No. 4,087,622, CA 2128154, CA 2128161 and U.S. Pat. No. 5,422,329.
Methods for the preparation of non-shell type catalysts are described in, for example, U.S. Pat. No. 3,743,607, GB 1333449, U.S. Pat. No. 3,939,199, U.S. Pat. No. 4,668,819, EP 330853, EP 403950, EP431478 and CA 2071698.
U.S. Pat. No. 5,336,802 describes a method for the pre-treatment of palladium-gold catalysts in which the catalyst is heated in the presence of an oxidising agent such as air at a temperature at least sufficient to partially oxidise the palladium; the oxidising agent is withdrawn and an inert gas such as nitrogen is introduced; the catalyst is then heated again at a temperature up to 500° C. in the presence of a reducing agent such as hydrogen or ethylene. The process described therein is illustrated with a “conventional catalyst containing nominally 1% palladium and 0.5% gold”.
It is known that the activity for vinyl acetate production of supported palladium catalysts declines with use. If the catalyst's activity and hence the process productivity declines to a commercially unacceptable level, it is necessary to regenerate and/or replace the catalyst. Deactivation of vinyl acetate catalysts is described by Abel et al. in Chem. Eng. Technol. 17 (1994) 112-118.
Merely increasing the amount of palladium in the catalyst to increase the lifetime of the catalyst presents a problem in that the initial activity of the catalyst may be too high for safe and/or controllable operation on an industrial scale, for example, due to the limited heat removal capacity of the plant.
There remains a need for a process for the preparation of a supported palladium catalyst for use in the production of vinyl acetate which overcomes this problem.
Thus, according to the present invention, there is provided a process for the production of vinyl acetate which process comprises contacting ethylene, acetic acid and an oxygen-containing gas with a supported palladium catalyst prepared by a process comprising the steps: (a) impregnating a catalyst support with a palladium compound, (b) converting the palladium compound to substantially metallic palladium, and (c) sintering, the supported palladium at a temperature of greater than 500° C.
The present invention solves the technical problem defined above by sintering the palladium on the support at a temperature of greater than 500° C.
Without wishing to be bound by any theory it is believed that this sintering step causes palladium metal particle growth which decreases the initial activity of the catalyst. Thus, catalysts having, a high palladium concentration but a commercially acceptable initial activity may be prepared by the process according to the present invention and such catalysts have a longer commercially useful life than conventional catalysts. The sintering step also increases the average pore size of silica supports. The catalysts of the present invention have also been found to be less susceptible to the adverse effects of excess concentration of promoter such as potassium acetate.
The sintering step (c) is preferably performed using a reducing gas, but can be performed in the presence of an oxidising gas or in an inert gas. Suitable reducing gases are hydrogen and carbon monoxide. A suitable oxidising gas is oxygen. These may be diluted with an inert gas. Suitable inert gases for use alone or in conjunction with oxidising or reducing gases are nitrogen, carbon dioxide and helium. Suitable temperatures for the sintering step are from greater than 500 to 1000° C. with preferred temperatures being in the range 650-1000° C. Preferred times for the sintering step are between 1 and 24 hours. If an oxidising gas is used then the catalyst needs to be subsequently reduced. The catalyst can be purged with an inert gas prior to sintering and during the heat-up period (for safety) and during cool-down (to less than 100° C., more preferrably to less than 60° C.) to prevent any redispersion of the palladium. Any suitable or practicable heat-up and cool-down rates can be used. The sintering step (c) on a commercial scale can be performed in a tower or vessel capable of fulfilling the process conditions outlined above. The catalyst can be agitated by the gas flow during the process. A rotary screw furnace can be used. On the laboratory scale, a horizontal or vertically mounted tube in an electric furnace can be used provided that gas-solid contact is efficient (length/diameter will need to be considered). Pre-heating of the gas stream may be required. The time and temperature of the sintering step are related; the higher the temperature, the shorter the time required. Those skilled in the art will be able to adapt these parameters to fit the scale of operations. Typically the sintering step (c) causes palladium metal particle growth from 3-4 nm in diameter to 8-15 nm in diameter.
The conversion of the palladium compound to substantially metallic palladium in step (b) may be achieved by a reduction step which can immediately precede the sintering step (c) and by performing the two process steps in the same equipment.
The catalyst preparation process of the present invention may be used for the preparation of uniformly impregnated or shell impregnated catalysts, for use in fluid bed or fixed bed processes for the production of vinyl acetate.
The catalyst preparation process of the present invention may be used to prepare catalysts having high palladium concentrations, for example greater than 0.5% by weight, preferably greater than 1% by weight based upon the total weight of the catalyst The palladium concentration may be as high as 5% by weight for fluid bed or as high as 10% by weight for fixed bed applications. The initial activity of a supported palladium catalyst having high palladium concentration, if prepared by a conventional process, would be expected to be very high and might even be so high as to be unsafe and/or uncontrollable if used on a commercial scale. However, when prepared by the process of the present intentional, the initial activity of the catalyst is reduced compared to that of a conventionally prepared catalyst, whereas the high palladium concentration results in commercially acceptable activity for the extended lifetime of the catalyst.
For the preparation of both shell impregnated and uniformly impregnated catalysts, suitable catalyst supports may comprise porous silica, alumina, silica/alumina, titania, zirconia or carbon, preferably silica. Suitably, the support may have a pore volume from 0.2 to 3.5 ml per gram of support, a surface area of 5 to 800 m
2
per gram of support and an apparent bulk density of 0.3 to 1.5 g/ml. For catalysts used in fixed bed processes the support typically has dimensions of 3 to 9 mm. For catalysts used in fixed bed processes the support typically may be spheric, tablet, extrudate, pill shaped or any suitable shape. For catalysts used in fluid bed processes the support typically may have a particle size distribution such that at least 60% of the catalyst particles have a particle diameter of below 200 microns, preferably at least 50% less than 105 microns and no more than 40% of the catalyst particles have a diameter less than 40 microns.
In step (a) the support is preferably impregnated with a palladium compound in a suitable solvent. Suitable solvents may be water, carboxylic acids such as acetic acid, benzene, toluene, alcohols such as methanol or ethanol, nitriles such as acetonitril
Couves John William
Kitchen Simon James
BP International Limited
Faye Sharpe, Fagan, Minnich & McKee LLP
Shippen Michael L.
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